Acute renal failure (ARF) is a common ailment in patients admitted to general medical-surgical hospitals. Approximately half of the patients who develop ARF die, and survivors face marked increases in morbidity and prolonged hospitalization [1]. Early diagnosis is generally believed to be critical, because renal failure is often asymptomatic and typically requires careful tracking of renal function markers in the blood. Dynamic monitoring of renal function of patients is highly desirable in order to minimize the risk of acute renal failure brought about by various clinical, physiological and pathological conditions [2-6]. Such dynamic monitoring is particularly important in the case of critically ill or injured patients, because a large percentage of these patients tend to face the risk of multiple organ failure (MOF) potentially resulting in death [7, 8]. MOF is a sequential failure of the lungs, liver and kidneys and is incited by one or more of acute lung injury (ALI), adult respiratory distress syndrome (ARDS), hypermetabolism, hypotension, persistent inflammatory focus and sepsis syndrome. The common histological features of hypotension and shock leading to MOF generally include tissue necrosis, vascular congestion, interstitial and cellular edema, hemorrhage and microthrombi. These changes generally affect the lungs, liver, kidneys, intestine, adrenal glands, brain and pancreas in descending order of frequency [9]. The transition from early stages of trauma to clinical MOF generally corresponds with a particular degree of liver and renal failure as well as a change in mortality risk from about 30% up to about 50% [10].
Traditionally, renal function of a patient has been determined using crude measurements of the patient's urine output and plasma creatinine levels [11-13]. These values are frequently misleading because such values are affected by age, state of hydration, renal perfusion, muscle mass, dietary intake, and many other clinical and anthropometric variables. In addition, a single value obtained several hours after sampling is difficult to correlate with other important physiologic events such as blood pressure, cardiac output, state of hydration and other specific clinical events (e.g., hemorrhage, bacteremia, ventilator settings and others).
With regard to conventional renal monitoring procedures, an approximation of a patient's glomerular filtration rate (GFR) can be made via a 24 hour urine collection procedure that (as the name suggests) typically requires about 24 hours for urine collection, several more hours for analysis, and a meticulous bedside collection technique. Unfortunately, the undesirably late timing and significant duration of this conventional procedure can reduce the likelihood of effectively treating the patient and/or saving the kidney(s). As a further drawback to this type of procedure, repeat data tends to be equally as cumbersome to obtain as the originally acquired data.
Occasionally, changes in serum creatinine of a patient must be adjusted based on measurement values such as the patient's urinary electrolytes and osmolality as well as derived calculations such as “renal failure index” and/or “fractional excretion of sodium.” Such adjustments of serum creatinine undesirably tend to require contemporaneous collection of additional samples of serum and urine and, after some delay, further calculations. Frequently, dosing of medication is adjusted for renal function and thus can be equally as inaccurate, equally delayed, and as difficult to reassess as the measurement values and calculations upon which the dosing is based. Finally, clinical decisions in the critically ill population are often equally as important in their timing as they are in their accuracy.
Thus, there is a need to develop improved compositions, devices and methods for measuring renal function (e.g., GFR) using non-ionizing radiation. The availability of a real-time, accurate, repeatable measure of renal excretion rate using exogenous markers under a variety of circumstances would represent a substantial improvement over any currently available or widely practiced method. Moreover, since such an invention would depend heavily on the renal elimination of the exogenous marker(s), the measurement would ideally be absolute and would, thus, preferably require little or no subjective interpretation based on age, muscle mass, blood pressure and the like. Indeed, such an invention would enable assessment of renal function under particular circumstances at particular moments in time.
It is known that hydrophilic, anionic substances are generally capable of being excreted by the kidneys [14]. Renal clearance typically occurs via two pathways: glomerular filtration and tubular secretion. Tubular secretion may be characterized as an active transport process, and hence, the substances clearing via this pathway typically exhibit specific properties with respect to size, charge and lipophilicity.
Most of the substances that pass through the kidneys are filtered through the glomerulus (a small intertwined group of capillaries in the malpighian body of the kidney). Examples of exogenous substances capable of clearing the kidney via glomerular filtration (hereinafter referred to as “GFR agents”) are shown in FIG. 1 and include creatinine (1), o-iodohippuran (2), and 99mTc-DTPA (3) [15-17]. Examples of exogenous substance that is capable of undergoing renal clearance via tubular secretion include 99mTc-MAG3 (4) and other substances known in the art [15, 18, 19]. 99mTc-MAG3 (4) is also widely used to assess renal function though gamma scintigraphy as well as through renal blood flow measurement. As one drawback to the substances illustrated in FIG. 1, o-iodohippuran (2), 99mTc-DTPA (3) and 99mTc-MAG3 (4) include radioisotopes to enable the same to be detected. Even if non-radioactive analogs (e.g., such as an analog of o-iodohippuran (2)) or other non-radioactive substances were to be used for renal function monitoring, such monitoring would require the use of undesirable ultraviolet radiation for excitation of those substances.
Currently, no reliable, continuous, repeatable method for the assessment of specific renal function using a non-radioactive, exogenous renal agent is commercially available. Among the non-radioactive methods, fluorescence measurement tends to offer the greatest sensitivity. In principle, there are two general approaches for designing fluorescent renal agents. The first approach would involve enhancing the fluorescence of known renal agents that are intrinsically poor emitters (e.g. lanthanide metal complexes) [21, 22], and the second approach would involve transforming highly fluorescent dyes (which are intrinsically lipophilic) into hydrophilic, anionic species to force them to clear via the kidneys.
Accordingly, it would be quite desirable to transform highly fluorescent dyes into hydrophilic, anionic species. More particularly, it would be quite desirable to identify appropriate, small, fluorescent molecules and render such molecules hydrophilic. Examples of dyes capable of absorbing light in the visible and/or NIR regions are shown in FIG. 2. These dyes are often relatively large in size, contain multiple aromatic rings, and are highly lipophilic compared to the structures shown in FIG. 1. Large lipophilic molecules almost always clear via the hepatobiliary system and do not readily clear via renal pathways. For example, FIG. 3 shows that tetrasulfonated cyanine dye (8 of FIG. 2) exhibits a poor rate of clearance from the blood. In attempts to circumvent this problem, some dyes have been conjugated to polyanionic carriers [23, 24]. Although these dye-polymer conjugates generally possess acceptable renal clearance properties, such polymeric compounds have other drawbacks such as polydispersity, manufacturing and quality control issues, and the provocation of undesired immune responses that may preclude their use as diagnostic and/or therapeutic substances. Accordingly, development of small, hydrophilic dyes is quite desirable to enable enhanced measurement of renal functioning and clearance.